1. Field of the Invention
The present invention relates to a color mixing prevention method and device for a field-sequential color television camera that uses a rotating filter wheel with side-by-side color filters to sequentially filter imaging light. The present invention also relates to a color balance setting method and device for a field-sequential color television camera that maximizes use of the fields and adjusts the time intervals for charge accumulation to achieve color balance in lieu of complexities in the signal executing circuit system.
2. Description of Related Art
In general, with color television cameras that use a solid state imaging element or the like, it is necessary to adjust the white balance in order to correct discrepancies in the color balance that occur through differences in color temperature or the like from the light source. This type of white balance adjustment has conventionally imaged a white object, for example, or taken light from a white light source into the camera, and adjusted the gain of each color through a white balance circuit located in the movie signal executing circuit of the camera. It has thus been possible, through adjusting the white balance, to image a white object, for example, with the proper degree of whiteness.
FIG. 6 shows the action timing of a prior art field-sequential color television camera that uses a solid state imaging element and a color separating color filter. With the field-sequential color television camera that corresponds to FIG. 6, one revolution of a rotary color filter with the three filter domains of G (green), B (blue), and R (red) is synchronized with the time interval for one cycle of 9 field divisions of the solid state imaging element. In the time interval from field 1 to field 3 the green field domain passes the imaging surface of the solid state imaging element. The blue domain passes during the interval from fields 4-6, and the red domain passes during the interval from fields 7-9. The solid state imaging element does not accumulate light from the object being photographed during all of the field intervals, but transmits the charge accumulated at the end of the fields to the surface image executing circuit of the camera. In the surface image signal executing circuit, a signal corresponding to the transmitted charge for each color is recorded, the signals recorded for each color being composed according to the desired ratios, and color image signals being formed and output.
With the above-mentioned prior-art field-sequential color television camera, two different color signals are projected onto the imaging surface and accumulated during the interval TM3, during which the color boundaries of the color separating filter pass over the imaging surface of the imaging element, resulting in color mixing. Any charge accumulating during the color mixing interval is not useful because it results from two different colors. With prior-art field-sequential color television cameras, the surface image accumulation is not carried out during the entire field time interval of the solid state imaging element when the field time interval includes color mixing. The surface image signals of fields that include the color mixing time interval TM3 are discarded, and ultimately cannot be used in the formation of the color image signal. For example, referring to FIG. 6, the data from fields 1, 4 and 7 would be discarded. U.S. Pat. No. 4,851,899, the disclosure of which is incorporated herein by reference, discloses such a prior art system.
Therefore, high-speed image input cannot be performed because the imaging signals from fields that include color mixing, during which the color boundaries of the color separating filter pass over the imaging surface, are ignored, and only the imaging signals from the other fields are used.
Additionally, such color television cameras require white balance circuits to adjust the gain of each color in the image signal executing circuit for the white balance. Such circuits complicate the construction of the camera circuits and hinder the development of smaller, lighter color television cameras. Because the signal level of image signals prior to the white balance execution in a prior-art color television camera varies widely with each color and because the signal executing circuits cross a wide dynamic range and must move stably and precisely, the circuits becomes very complicated and costly.